The present invention relates to a process for the preparation of polyethylenes, having a multimodal molecular weight distribution, more particularly a bimodal or trimodal molecular weight distribution and exhibiting outstanding mechanical properties for blow-molding, film and pipe applications.
Polyolefins such as polyethylenes which have high molecular weight generally have improved mechanical properties over their lower molecular weight counterparts. However, high molecular weight polyolefins can be difficult to process and can be costly to produce. Polyolefins having a bimodal molecular weight distribution are desirable because they can combine the advantageous mechanical properties of high molecular weight fraction with the improved processing properties of the low molecular weight fraction.
For many HDPE applications, polyethylene with enhanced toughness, strength and environmental stress cracking resistance (ESCR) is important. These enhanced properties are more readily attainable with high molecular weight polyethylene. However, as the molecular weight of the polymer increases, the processability of the resin decreases. By providing a polymer with a broad or bimodal MWD, the desired properties that are characteristic of high molecular weight resin are retained while processability, particularly extrudibility, is improved.
There are several methods for the production of bimodal or broad molecular weight distribution resins: melt blending, reactor in series configuration, or single reactor with dual site catalysts. Use of a dual site catalyst for the production of a bimodal resin in a single reactor is also known.
Chromium catalysts for use in polyolefin production tend to broaden the molecular weight distribution and can in some cases produce bimodal molecular weight distribution but usually the low molecular part of these resins contains a substantial amount of the comonomer. Whilst a broadened molecular weight distribution provides acceptable processing properties, a bimodal molecular weight distribution can provide excellent properties. In some cases it is even possible to regulate the amount of high and low molecular weight fraction and thereby regulate the mechanical properties.
Ziegler Natta catalysts are known to be capable of producing bimodal polyethylene using two reactors in series. Typically, in a first reactor, a low molecular weight homopolymer is formed by reaction between hydrogen and ethylene in the presence of the Ziegler Natta catalyst. It is essential that excess hydrogen be used in this process and, as a result, it is necessary to remove all the hydrogen from the first reactor before the products are passed to the second reactor. In the second reactor, a copolymer of ethylene and hexene is made so as to produce a high molecular weight polyethylene.
Metallocene catalysts are also known in the production of polyolefins. For example, EP-A-0619325 describes a process for preparing polyolefins such as polyethylenes having a multimodal or at least bimodal molecular weight distribution. In this process, a catalyst system which includes at least two metallocenes is employed. The metallocenes used are, for example, a bis(cyclopentadienyl) zirconium dichloride and an ethylene bis(indenyl) zirconium dichloride. By using the two different metallocene catalysts in the same reactor, a molecular weight distribution is obtained which is at least bimodal.
Polyethylene resins are known for the production of pipes. Pipe resins require high resistance against slow crack growth as well as resistance to rapid crack propagation yielding impact toughness. There is a need to improve in the performance of currently available pipe resins.
EP-A-0571987 discloses a process for producing an ethylenic polymer composition using multistage polymerisation. The catalyst comprises, as principal components, a transition metal compound, a compound capable of reacting with the transmission metal compound to form an ionic complex and an organoaluminium compound.
EP-A-0600482 discloses the production of a resin composition of laminates which includes two polyethylene components, one of the components being prepared using a metallocene catalyst comprising ethylene-bis(4,5,6,7-tetrahydroindenyl) zirconium dichloride.
EP-A-0575123 discloses an ethylene polymer composition which may be produced using a metallocene catalyst.
EP-A-0605952 discloses an ethylene/alpha-olefin comonomer composition and a polymerisation process therefor which employs at least two kinds of specific metallocene compounds.
EP-A-0735090 discloses a polyethylene resin composition which is produced by physical blending of three polyethylene components.
EP-A-0791627 discloses a polyethylene resin composition which is produced using a metallocene catalyst.
WO-A-95/26990 discloses a process for producing polymers of multi-modal molecular weight distributions using metallocene catalysts.
The present invention aims to overcome the disadvantages of the prior art.
The present invention provides a process for the preparation of polyethylene resins having a multimodal molecular weight distribution which comprises:
(i) contacting ethylene monomer and a comonomer comprising an alpha-olefin having from 3 to 10 carbon atoms with a first catalyst system in a first reactor under first polymerisation conditions to produce a first polyethylene having a first molecular weight, an HLMI of not more than 0.5 g/10 min and a first density of not more than 0.925 g/ml and the first catalyst system comprising (a) a metallocene catalyst comprising a bis tetrahydroindenyl compound of the general formula (IndH4)2Rxe2x80x3MQ2 in which each Ind is the same or different and is indenyl or substituted indenyl, Rxe2x80x3 is a bridge which comprises a C1-C20 alkylene radical, a dialkyl germanium or silicon or siloxane, or an alkyl phosphine or amine radical, which bridge is substituted or unsubstituted, M is a Group IVB transition metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; and (b) a cocatalyst which activates the catalyst component;
(ii) providing a second polyethylene having a second lower molecular weight and a second higher density than the first polyethylene, the second polyethylene having been produced using a catalyst other than the bis tetrahydroidenyl compound; and
(iii) mixing together the first and second polyethylenes to form a polyethylene resin having a multimodal molecular weight distribution.
Optionally, the polyethylene resin is a pipe resin having an HLMI of from 3 to 10 g/10 min and a density of from 0.95 to 0.96 g/ml.
In this specification, the HLMI is measured by the procedures of ASTM D 1238 using a load of 21.6 kg at a temperature of 190xc2x0 C.
The present invention further provides a process for the preparation of polyethylene resins having a bimodal molecular weight distribution which comprises:
(i) contacting ethylene monomer and a comonomer comprising an alpha-olefin having from 3 to 10 carbon atoms with a first catalyst system in a first reactor under first polymerisation conditions to produce a first polyethylene having a first molecular weight, an HLMI of not more than 0.5 g/10 min and a first density of not more than 0.925 g/ml and the first catalyst system comprising (a) a metallocene catalyst comprising a bis tetrahydroindenyl compound of the general formula (IndH4)2Rxe2x80x3MQ2 in which each Ind is the same or different and is indenyl or substituted indenyl, Rxe2x80x3 is a bridge which comprises a C1-C20 alkylene radical, a dialkyl germanium or silicon or siloxane, or an alkyl phosphine or amine radical, which bridge is substituted or unsubstituted, M is a Group IVB transition metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; and (b) a cocatalyst which activates the catalyst component;
(ii) providing a second polyethylene having a second lower molecular weight and second higher density than the first polyethylene; and
(iii) mixing together the first and second polyethylenes to form a polyethylene resin having a bimodal molecular weight distribution, an HLMI of from 3 to 10 g/10 min and a density of from 0.95 to 0.96 g/ml.
The present invention yet further provides a process for the preparation of a linear low density polyethylene resin having a bimodal molecular weight distribution which comprises:
(i) contacting ethylene monomer and a comonomer comprising an alpha-olefin having from 3 to 10 carbon atoms with a catalyst system in a first reactor under first polymerisation conditions to produce a first polyethylene and the catalyst system comprising (a) a metallocene catalyst comprising a bis tetrahydroindenyl compound of the general formula (IndH4)2Rxe2x80x3MQ2 in which each Ind is the same or different and is indenyl or substituted indenyl, Rxe2x80x3 is a bridge which comprises a C1-C20 alkylene radical, a dialkyl germanium or silicon or siloxane, or an alkyl phosphine or amine radical, which bridge is substituted or unsubstituted, M is a Group IVB transition metal or vanadium and each Q is hydrocarbyl having 1 to 20 carbon atoms or halogen; and (b) a cocatalyst which activates the catalyst component;
(ii) transferring the catalyst system and the first polyethylene to a second reactor serially connected to the first reactor and in the second reactor contacting ethylene monomer and a comonomer comprising an alpha-olefin having from 3 to 10 carbon atoms with the catalyst system under second polymerisation conditions to produce a product comprising a second polyethylene having a different molecular weight distribution than that of the first polyethylene; and
(iii) blending together the first and second polyethylenes in the second reactor to form a linear low density polyethylene resin having a bimodal molecular weight distribution and a density of around 0.925 g/ml.
The first polyethylene may be monomodal or bimodal, the second polyethylene may have a monomodal molecular weight distribution and may have been produced using a metallocene catalyst, a Ziegler-Natta catalyst or a chromium-oxide based catalyst. Alternatively, the second polyethylene may have a bimodal molecular weight distribution and has been produced using one or two of those different catalyst systems. The first and second polyethylenes may be mixed together with a third polyethylene to provide a trimodal molecular weight distribution in the resultant polyethylene resin. The third polyethylene may be produced using a metallocene catalyst, a Ziegler-Natta catalyst or a chromium-oxide based catalyst.
The first and second polyethylenes may be mixed by chemical blending or physical blending. For chemical blending, the first and second polyethylenes are made in two serially connected reactors using a common metallocene catalyst (a), or three serially connected reactors for making a polyethylene resin having a trimodal molecular weight distribution, in which a third polyethylene is chemically blended with the first and second polyethylenes. In an alternative arrangement, the first and second polyethylenes may be chemically blended as foresaid, and then physically blended with a third polyethylene to produce a trimodal molecular weight distribution. In further alternative arrangements, the polyethylene resin has a bimodal molecular weight distribution and is produced by physically blending the first and second polyethylenes together or alternatively the polyethylene resin has a trimodal molecular weight distribution and is produced by physically blending together the first, second and third polyethylenes. Alternatively, a trimodal polyethylene may be produced in three reactors in series.
In the embodiments of the invention, the low density fraction produced using the metallocene catalyst (a) comprises at least 15 wt % of the resultant polyethylene resin.
The present invention is predicated on the discovery that the use of the particular bisetrahydroindenyl metallocene catalyst component (a) enables a high molecular weight linear low density polyethylene fraction e.g. in a pipe resin to be produced, with that fraction having a very narrow molecular weight distribution. This yields both improved slow and rapid crack propagation properties as a result of a high and uniform level of comonomer distribution in the low density fraction, with a density not more than 0.925 g/ml, compared to somewhat higher low density fractions achievable by Ziegler-Natta or chromium based catalysts, particularly when used in a slurry loop process. Thus the use of this metallocene catalyst enables precise control of the molecular weight distribution and density of the high molecular weight fraction of a pipe resin, yielding improved mechanical properties and processability. The HLMI of the high molecular weight, low density fraction is very low. The values of HLMI are representative of the high molecular weight of the fraction. Typically, overall the multimodal pipe resins of the present invention have a density of from 0.95 to 0.96 g/ml with an HLMI of from 3 to 10 g/10 min. The pipe resin consists not only of the high molecular weight fraction, but also a low molecular weight fraction whereby the pipe resin as a whole has a multimodal, for example a bimodal, molecular weight distribution.
The provision of such a multimodal distribution yields a combination of improved mechanical properties of the pipe resin, without compromising the processability.
In accordance with this invention, the low molecular weight fraction of the polyethylene resin for the production of pipes may be constituted by a second polyethylene which typically has a monomodal or bimodal molecular weight distribution and is produced by ethylene homo and/or copolymerisation in the presence of a metallocene catalyst system and/or a Ziegler-Natta catalyst system and/or a chromium-oxide based catalyst system.
A third polyethylene resin may also be provided which has a monomodal or bimodal molecular weight distribution and is produced using a metallocene catalyst system and/or a Ziegler-Natta catalyst system and/or a chromium-oxide based catalyst system but which has a different density and molecular weight distribution from that of the second polyethylene resin.
The first and second, and optionally third, polyethylenes constitute separately produced resins which may then be physically and/or chemically blended (in this case using a plurality of reactors in series) to form the composite polyethylene resin having a multimodal molecular weight distribution. The production of the polyethylene comprising the lower molecular weight fraction of the composite resin can be controlled to give the desired processing properties for the pipe resin. It has been shown that the combination of low branching (ideally no branching) in the low molecular part of the resin and high comonomer incorporation in the high molecular part significantly improves the resin properties with respect to resistance to slow crack growth and impact strength which are important properties for pipe resins.
In one preferred aspect of the invention, the first and second polyethylenes are physically blended to form a linear low density polyethylene (LLDPE) resin and the first and second polyethylenes are produced using the metallocene catalyst (a). Preferably, each of the first and second polyethylenes has a density of around 0.925 g/ml. Alternatively, in a preferred embodiment the first polyethylene has a density of around 0.905 g/ml and the second polyethylene has a density of around 0.930 g/ml. More preferably, the physical blend comprises substantially equal parts by weight of the first and second polyethylenes and has an HLMI of from 7 to 7.5 g/10 min. In another preferred aspect of the invention, the first and second polyethylenes are chemically blended and are produced using the metallocene catalyst (a). Preferably, the first and second polyethylenes together have a bimodal molecular weight distribution in the LLDPE.